Depending on when Koum's last day at Facebook is, he might lose out on as much as $1 billion in stock awards, Bloomberg reported, citing regulatory filings.

Facebook bought WhatsApp in 2014 for $19 billion, making Koum and his cofounder Brian Acton billionaires.

Usually, stock awards after a merger are distributed on a four-year "vesting cliff" — if you last all four years, you get your entire stock grant. Both Koum and Acton, who quit Facebook in September, will have left the company within that time.

According to Bloomberg, Koum still has three vesting dates through the end of the year: one in May, one in August, and the final one in November. The value of the shares he would get if he still worked at Facebook is just under $1 billion at Facebook's current share price.

Koum will also leave Facebook's board of directors, the company confirmed in a filing with the Securities and Exchange Commission.

Don't cry for Koum, though — he has already sold about $7.1 billion worth of Facebook shares, according to Courtney Yu, the director of research at Equilar, which tracks board compensation. Koum still holds over 11 million shares of Facebook, worth over $2 billion as of Tuesday.

So he's got plenty of money, even if he is leaving stock on the table.

"It's been almost a decade since Brian and I started WhatsApp, and it's been an amazing journey with some of the best people. But it is time for me to move on. I've been blessed to work with such an incredibly small team and see how a crazy amount of focus can produce an app used by so many people all over the world.

"I'm leaving at a time when people are using WhatsApp in more ways than I could have imagined. The team is stronger than ever and it'll continue to do amazing things. I'm taking some time off to do things I enjoy outside of technology, such as collecting rare air-cooled Porsches, working on my cars and playing ultimate frisbee. And I'll still be cheering WhatsApp on — just from the outside. Thanks to everyone who has made this journey possible."